**Only option (d) is correct ie. the speed is 1.0 m/s when the acceleration is half the initial value.**

**Explanation:**

**(A) **

**(B) ****The terminal speed is the constant speed when acceleration is zero. Thus **

**0 = 6.0-3v**

**v = 2 m/s**

**(C)** At t=0, v=0; therefore initial acceleration a = 6.0-0 = 6.0 m/s^{2}

**(D) **Acceleration is half that of initial value = 6.0/2 = 3.0m/s^{2}

This gives 3 = 6-3v(t)

**or v(t) = 1m/s**